This invention relates generally to clinical electronic thermometers for measuring body temperature by use of a temperature sensing transducer. More particularly, it relates to an improved electronic thermometer including, as the temperature sensing transducer, a thermistor which has a low thermal mass for providing a fast response time.
Electronic clinical thermometers are generally divided into two categories--steady state and predictive. The steady state or direct reading thermometer provides for a gradual rise to the maximum measurable temperature as does the common mercury thermometer. However, the temperature measurement is obtained in less time than the measurement obtained by the mercury thermometer. The response time usually required for the steady state thermometer is about sixty (60) seconds as opposed to three (3) to four (4) minutes for the mercury device. The predictive electronic clinical thermometer measures the rate of increase in temperature over, typically, ten (10) seconds and thereafter predicts or estimates a final temperature value based on the rate determined during the initial ten second measurement period.
Predictive electronic thermometers, such as the IVAC 811/821 System, which estimate the final temperature, i.e. before the transducer has actually reached a temperature equalized with that of the environment, have been implemented to attempt to reduce the time necessary for obtaining a final temperature reading. This type of thermometer may employ an electronic race circuit or some other type of compensation circuit which attempts to offset the actual measured temperature by a factor intended to compensate for the thermal time constant of the transducer. In the alternative, these thermometers may use numerical techniques provided as software algorithms to a built-in microprocessor to predict final temperature readings. Nevertheless, the problems associated with predicting thermometers are that these compensation arrangements and numerical predicting techniques do not accurately simulate the individually different physiological characteristics and responses of different human beings. These thermometers also present problems in their calibration. Generally, the thermometer is calibrated in an oil or water bath which has a much better thermal conductivity than the mouth or other body cavity. The compensation circuits react differently in oil or water baths rather than in the body cavity of the human being to make accurate calibration for human use difficult or impossible.
Steady state clinical thermometers directly measure temperature without use of compensation or predicting arrangements. Steady state thermometers can be accurately calibrated since the temperature they measure is the same temperature as the physiological environment. The major disadvantage of prior art steady state thermometers is that they require a predetermined time of contact with the environment during which the temperature of the thermometer equalizes with that of the environment.
Many electronic thermometers use thermistors as temperature sensing transducers for measuring body temperature. In clinical thermometers, a thermistor is attached to a probe and the probe is inserted into a body cavity such as the mouth of the human being whose temperature is being measured. Once inserted, the probe must remain in place for a sufficient time to permit the thermistor to be heated to the temperature of its environment, i.e. body tissue. Measuring the temperature before the thermistor (or, more accurately, the probe in the vicinity of the thermistor) has reached a temperature equal to that of the environment will result in an inaccurate, typically low, temperature reading. Probe preheators have been employed with thermistors to speed the response by raising the local temperature of the probe in the vicinity of the thermistor. This technique has been disclosed in the following patents:
______________________________________ U.S. PAT. NO. INVENTOR ISSUE DATE ______________________________________ 3,025,706 Oppenheim March 20, 1962 3,729,998 Mueller et al. May 1, 1973 3,485,102 Glick December, 23, 1979 3,893,058 Keith July 1, 1975 3,951,003 Adams October 28, 1975 4,166,451 Salera September 4, 1979 4,133,208 Parlanti January 9, 1979 ______________________________________
The amount of time that the probe must remain in the body cavity depends on the thermal time constant of the thermistor. The thermal time constant is a measure of the time required for a particular object to absorb or expel a quantity of heat and, therefore, represents a measure of the speed with which heat energy can be transferred by the object. The thermal time constant of the thermistor is a function of its thermal mass. The time constant of the thermistor is generally less than the time constant of the probe--on the order of ten (10) times less. However, because the thermistor, as disclosed in the Oppenheim patent for example, is surrounded by the probe, the time required for the thermistor temperature to reach steady state body temperature is dependent upon the thermal time constant of the probe. This probe/thermistor configuration could result in a response time of one (1) minute or more.
A clinical electronic thermometer, designed to overcome problems associated with prior art electronic thermometers such as inaccuracy due to the insulating characteristics of the probe, long thermal time constants and the cooling effect of the mass of the probe on the body tissue, is disclosed in U.S. Pat. No. 4,174,631 which issued to Hammerslag on Nov. 20, 1979. The Hammerslag invention includes a thermocouple having two dissimilar fine metal wires that make direct contact with body tissue for providing a true (not predictive) temperature reading in less than one minute. Specifically, the Hammerslag patent shows a thermocouple (thermoelectric) type transducer having a low thermal mass for providing a fast response which is achieved by supporting the thermocouple leads in a hollow tube of plastic material and causing the thermocouple junction to bridge the open end of the tube so that the thin wires and junction are exposed for direct contact with the tissue of the human being whose body temperature is to be measured. However, since the thermocouple leads are exposed, they are readily subject to damage. Also, because the tube or probe is open-ended, it may become contaminated due to entry of saliva and foreign particles.
Another type of thermometer having exposed leads is shown in U.S. Pat. No. 4,250,751 issued to Holzhacker et al. on Feb. 17, 1981. Another temperature sensing probe is disclosed and claimed in a pending patent application, U.S. Ser. No. 309,667, in the name of Schwarzchild and having common assignee herewith, now U.S. Pat. No. 4,411,535 issued Oct. 25, 1983.
Thermistors have been preferred over thermocouples for providing temperature measurements in the medical environment because they exhibit greater sensitivity in the temperature range of interest and because no reference junction is required as in the thermocouple. Also, when using thermocouples, measurement errors may be caused by, for example, the Peltier effect wherein one thermocouple junction is warmed while the other is cooled. This would not occur in thermistor-based temperature sensing probes.
Accordingly, one object of the invention is to provide an improved fast response clinical electronic thermometer having a probe for insertion into body cavities.
An object of the invention is to provide an improved fast response clinical electronic thermometer which includes both a probe and a thermal sensor assembly within the probe having very low thermal mass.
Another object of the invention is to provide for the thermal sensor assembly to make direct contact with the body tissue in the body cavity for obtaining a fast temperature measurement without the need for compensating arrangements such as probe preheators.
Another object of the invention is to provide a clinical thermometer that produces an accurate (.+-.0.1.degree. C.) temperature measurement in less than ten (10) seconds once the probe is positioned in the body cavity and makes contact with the body tissue.
Yet another object of the invention is to substantially eliminate the problem of contamination by providing a closed-end probe.
A further object of the invention is to minimize damage by enclosing the temperature sensing element and associated leads within the hollow tube-like probe body. A further object of the invention is to provide an improved construction for an inexpensive clinical thermometer.